Sawtooh electric field drift region structure for planar and trench power semiconductor devices

ABSTRACT

A lateral super junction JFET is formed from stacked alternating P type and N type semiconductor layers over a P-epi layer supported on an N+ substrate. An N+ drain column extends down through the super junction structure and the P-epi to connect to the N+ substrate to make the device a bottom drain device. N+ source column and P+ gate column extend through the super junction but stop at the P-epi layer. A gate-drain avalanche clamp diode is formed from the bottom the P+ gate column through the P-epi to the N+ drain substrate.

This patent application is a Continuation Application and claims thePriority Date of a co-pending application Ser. No. 16/888,768 filed onMay 23, 2020, and application Ser. No. 16/888,768 is a ContinuationApplication and claims the Priority Date of application Ser. No.16/153,567 filed on Oct. 5, 2018. Application Ser. No. 16/153,567 is aContinuation Application and claims the Priority Date of a co-pendingapplication Ser. No. 13/763,675 filed on Feb. 10, 2013. Application Ser.No. 13/763,675 is a Divisional Application and claim the Priority Dateof Application of another Application with a Ser. No. 12/799,810 filedby a common Inventor of this Application on Apr. 30, 2010 and now issuedinto U.S. Pat. No. 8,373,208 on Feb. 12, 2013. Application Ser. No.12/799,810 is a Continuation in Part (CIP) Application of applicationSer. No. 12/592,619 filed on Nov. 30, 2009 now issued into U.S. Pat. No.8,575,695 on Nov. 5, 2013.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The invention relates generally to semiconductor power devices. Moreparticularly, this invention relates to configurations and methods tomanufacture lateral power device including a super-junction structurewith an avalanche clamp diode formed between the drain substrate and thegate. The lateral super-junction structure reduces on-resistance, whilethe structural enhancements increase the breakdown voltage betweensubstrate and drain and improve unclamped inductive switching (UIS)performance

2. Description of the Prior Art

Conventional semiconductor power device such as the MOSFET power devicesimplemented with a super-junction structure can achieve performanceimprovements with significant reduction of the on-resistance while stillmaintaining a high breakdown voltage. However, the manufacturingtechnologies and device configuration for implementing thesuper-junction structures in the MOSFET devices are still confrontedwith manufacturability difficulties. The manufacturability and cost ofthe conventional vertical power devices designed for high voltageapplications implemented with super-junction structures are limited dueto the structural features that require numerous manufacturing processeswhich are time-consuming, complex, and expensive. The manufacturingprocesses according to current practice involve many sequential masking,implantation and epitaxial growth steps to build the vertical structure.Achieving a high density of alternately doped columns becomesprohibitive since it requires a direct increase in the number of thesesteps. Too many factors influence the accuracy of charge-balance betweenadjacent alternately doped columns, leading to narrow process margins asa high density of said columns is attempted. FIG. 1A shows a typicaldesign for a MOSFET vertical super junction device, as disclosed byTatsuhiko Fujihira in his paper “Theory of Semiconductor SuperjunctionDevices” published in Jpn. J. Appl. Phys. 36 (1997) pp. 6254-6262. It isdifficult and costly to manufacture the vertical charge-balancedalternately doped columns of Fig. A, especially at high densities

For these reasons, lateral JFET power devices with super junctionstructures formed with stacked horizontal layers of alternating dopantconductivity types overcome these difficulties. This device may beconfigured in cascade with a low voltage MOSFET to achieve thenormally-off operation of a conventional device. Coe discloses a lateralpower device in U.S. Pat. No. 4,754,310 with charge balanced superjunction structure configured with stacked horizontal layers ofalternating conductivity types extended between a source and draincolumn. Such structure of stacked horizontal layers can be efficientlymanufactured without the use of masks. However, a typical deviceconfiguration as shown in FIG. 1B is limited by its drain-substratebreakdown voltage and is further restricted due to the difficulty inimproving its unclamped inductive switching (UIS). FIG. 1C shows anexample of another lateral super junction device as disclosed byTatsuhiko Fujihira in the same paper as mentioned above, “Theory ofSemiconductor Superjunction Devices”. This device suffers from excessivechannel resistance in distributing the current into the stacked n-typeconduction paths.

Therefore, a need still exists in the art of power semiconductor devicedesign and manufacture to provide new device configurations andmanufacturing methods for forming the lateral power device such that theabove discussed problems and limitations can be resolved.

SUMMARY OF THE INVENTION

It is therefore an aspect of the present invention to provide a new andimproved device structure and manufacturing method to form the junctionfield effect transistor (JFET) and MOSFET power device on an P-epitaxiallayer functioning as an intermediate semiconductor layer over an Nsubstrate constituting a bottom semiconductor layer with lateral superjunction structure extended between deep N type columns functioning asthe source and drain columns, and a deep P+ column to function as theJFET gate. In this structure, the drain terminal is moved to thesubstrate in order to separate the two high current terminals toseparate planes to allow for better current spreading. This is achievedby making the drain trench deeper than that of source and gate, so thatthe drain terminal cuts through the P epitaxial region and contacts theN+ substrate. This configuration also results in the formation of aconcave or saddle N+ drain substrate-P-epitaxial N+ drain columnjunction that can be designed to achieve high substrate blockingvoltage. In addition, an N+-P-P+ gate-drain avalanche clamp diode isformed from the N+ drain substrate to the P-epitaxial to the P+ gatecolumn to provide a rugged high voltage diode that diverts the avalanchecurrent away from the superjunction layers of this device.

Another aspect of this invention is to provide a new and improved devicestructure and method to manufacture a normally off semiconductor powerdevice with a structure that integrates a low voltage MOSFET with thesuper-junction JFET. The low voltage MOSFET is disposed near the devicesurface with a configuration to achieve the cascode connection requiredto make the normally off switch.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodiment,which is illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross sectional view for showing a conventionalconfiguration of a vertical super junction power device.

FIG. 1B is a cross sectional view of a conventional lateral superjunction power device.

FIG. 1C is a cross section of a perspective view of a conventionallateral super junction power device.

FIG. 2 is a cross sectional view of a lateral super junction powerdevice of this invention.

FIGS. 3A to 3N are a series of cross sectional views showing a method offorming a lateral super junction power device of this invention.

FIGS. 3A-1 to 3C-1 area a series of cross sectional views showing analternative method of forming a lateral super junction power device ofthis invention.

FIGS. 4A to 4C are top views for showing possible layouts forintegrating a MOSFET with a lateral super junction power device of thisinvention.

FIGS. 5A to 5B are cross sections of perspective views for a MOSFETintegrated with a lateral super junction power device of this invention.

FIG. 5C is a top view of the device from FIGS. 5A and 5B.

FIGS. 5D to 5F are cross sectional views taken along cross section linesA-A, B-B and C-C of FIG. 5B.

FIG. 6A is a top view showing a stripe configuration of a lateral superjunction JFET of this invention.

FIGS. 6A-1 to 6A-3 are cross sectional views taken along cross sectionlines D-D and E-E of FIG. 6A.

FIGS. 6B and 6C are top views showing how a MOSFET may be integratedwith a JFET of FIG. 6A.

FIG. 6C-1 is a cross sectional view taken along cross section line F-Fof FIG. 6C.

FIG. 6D is a top view showing multiple rows of JFETs in stripeconfiguration.

FIG. 7A is a top view showing a closed cell configuration of a lateralsuper junction JFET of this invention.

FIG. 7A-1 is a cross sectional view taken along cross section lines G-Gof FIG. 7A.

FIGS. 7B and 7C are top views showing how a MOSFET may be integratedwith a JFET of FIG. 7A.

FIG. 7C-1 is a cross sectional view taken along cross section line G-Gof FIG. 7C.

FIGS. 8A to 8Q are a series of cross sectional views showing how to forma MOSFET integrated with a lateral super junction JFET of thisinvention.

DETAILED DESCRIPTION OF THE METHOD

FIG. 2 is a cross sectional view of a lateral super junctionhigh-voltage (HV) JFET device 100 having a bottom drain substrate of thepresent invention. The lateral JFET device 100 is supported on aP-Epitaxial layer 115 and an optional N buffer region 106 on an N+substrate 105 functioning as a drain. The lateral JFET device 100includes a drain metal electrode 110 disposed on the bottom of thesubstrate 105. The lateral power device 100 further includes an N sourcecolumn 120-S and an N drain column 120-D formed respectively as firstand second conductive columns disposed in a first trench and a secondtrench respectively on two opposite sides on the substrate. In thisembodiment, each of these source and drain trenches may comprise aconductive material in the trench such as a metal fill or polysiliconsurrounded by an N+ doped region 125. Of course, any other scheme may beutilized to form the N+ drain and source columns 120-D and 120-S. The N+source column 120-S contacts a source metal 120-S-M on the top surfacethrough the oxide 107. The drain 120-D disposed in the trench extendsdownwardly to reach the N+ drain substrate layer 105. The source trenchis etched down into the P epitaxial layer 115. A P+ doped region 165 isdisposed below the source 120-S to suppress activation of the parasiticNPN bipolar transistor formed from the N+ source column 120-S to theP-epitaxial layer 115 to the N+ substrate 105. The heavily doped P+region 165 in the P-epitaxial layer 115 greatly reduces the minoritycarrier lifetime to suppress the parasitic NPN transistor.

The lateral JFET device 100 further includes a P+ doped column 130disposed in another trench functioning as a gate of the lateral powerdevice. Like the N+ source and drain columns 120-S and 120-D, the P+gate column 130 comprises a trench filled with metal or polysiliconsurrounded by a P+ doped region 135. The P+ gate column 130 and theP-epitaxial layer 115, and the optional N-buffer region 106, are furtherdesigned to form an avalanche diode 121 to clamp the breakdown voltage.A super junction device is normally very vulnerable to damage fromavalanche breakdown. If avalanche breakdown occurs in the chargebalanced regions, the avalanche current tends to focus and magnify onthe small area in which it started, permanently damaging that area. Theavalanche diode 121 diverts the avalanche current path away from thecharge balanced super junction region so that it instead flows betweenthe bottom of the P+ gate column 130 and the N+ drain layer 105, thusincreasing the robustness of the device. The P+ gate column 130 isconnected to a gate metal 130-M on the top surface. The source and drain120-S and 120-D may also be formed with N+ doped polysilicon filling inthe trenches or may be formed with metal plug filling the trenches withN+ doped sidewalls. Alternatively, the trenches may be filled byepitaxial growth or formed by any other method. It is important that thesidewalls of the trench are formed with an N+ doped semiconductormaterial.

The lateral power device has a super junction structure formed asmultiple layers of alternating horizontal P doped layers 140 and N dopedlayers 150. These alternating P doped and N doped layers provide chargebalanced conduction channels between source 120-S and the drain 120-D.The electric current is conducted along a lateral direction with a gate130 that controls and switches on and off the power device. The gate 130in FIG. 1 is disposed at a different plane from the source and drain andtherefore does not cut off the lateral channels formed in thesuper-junction structure with the alternating N and P doped layers. Boththe gate column 130 and the source column 120-S should be formed to bedeeper than the super junction structure so as to have contact with theP epitaxial layer 115. The gate column 130 may be formed to the samedepth as the source column 120-S, or it may be formed deeper than sourcecolumn 120-S. Forming the gate column 130 deeper than source column120-S can help direct avalanche current to the gate column 130 andavalanche diode 121 rather than to the parasitic NPN transistor underthe source column 120-S. The schematic circuit of the lateralsuperjunction JFET device 100 shows the drain metal 110, source metal120-S-M and the gate metal 130-M as the device terminals. The avalanchediode 121 is formed between the gate and the drain.

FIGS. 3A to 3N are a series of cross sectional views for illustratingthe processing steps for manufacturing a JFET device with lateralsuper-junction structure of this invention. FIG. 3A shows a P+ substrate205 supporting a P-epitaxial (P-epi) layer 215 with a dopantconcentration and layer thickness designed to block a voltage of 600volts. Alternating P and N dopant layers 240 and 250 respectively areformed on top of P-epi layer 215 as charge balanced layers, e.g. byusing one of the processes described in FIGS. 10 to 12 of U.S.application Ser. No. 12/592,619. In FIG. 3B, a hard mask layer 222 isformed, e.g., through a thermal oxide process to grow an oxide layerwith a layer thickness of approximately 205 Angstroms followed by oxidedeposition to form thick oxide layer to function as a hard mask layer222. A photoresist mask is used to etch through the hard mask 222 andform opening 223. In FIG. 3C, a silicon etch process is carried out toopen the gate trenches 225 extending vertically through thesuper-junction structure of alternating P and N dopant layers 240 and250 to reach the P-epitaxial layer 215 having a depth of approximately60 micrometers and a width of about 10 micrometers. The gate trench 225may be formed with a slight taper of 88 degrees to improve themanufacturability of the device. In FIG. 3D, a P+ implant with boronions into the gate trench to form the P+ region 230 surrounding the gatetrenches 225. By way of example, the boron implants may compriseimplants having implant energy of about 40 keV, concentration of about5E15 cm⁻², and a tilt angle of about seven degrees and four 90-degreerotations and a perpendicular boron ion implant of the same ion flux. InFIG. 3E, a polysilicon lining layer 235 is first formed on the sidewallsand bottom surface of the gate trenches 225, then an oxide layer 241 isfilled into the gate trenches. The polysilicon layer may also be dopedP+ to reduce the resistance of the gate trench. Then a polysilicon andoxide etch back is carried out to remove the polysilicon and oxide abovethe top surfaces. The polysilicon layer 235 is optional, but can act asa convenient endpoint when etching or chemical mechanical polishing(CMP) back the oxide layer 241. The polysilicon 235 may also bedeposited undoped—later P type dopants from the P+ layer 230 willdiffuse into the polysilicon 235.

In FIG. 3F a second oxide layer 222′ is deposited on top of the firstoxide layer 222. In FIG. 3G, a source mask is applied to open trenchopenings 224 through the oxide layers 222 and 222′. In FIG. 3H, sourcetrenches 245-S are etched through the super-junction structure to adepth reaching the P-epi layer 215. In one embodiment the depth of thetrenches is approximately between 40 and 44 micrometers and having awidth of about ten micrometers. At any rate, the source trenches 245-Sshould end in the P-epi layer 215. Then the fabrication processesproceed with an angled N+ implant of about 7 degrees tilt followed by aperpendicular N+ implant to form N+ regions 255 surrounding thesidewalls and bottom of the trenches 245-S. In FIG. 3I, a perpendicularP+ implant is carried out to form the P+ regions 265 at the bottom ofthe trenches 245-S. In FIG. 3J, a Ti/TiN layer 260 is first depositedcovering the sidewalls, the bottom surface of the trenches and over thetop surface of the oxide 222′. Then, a thick metal layer 251 of 6-8micrometers is deposited to fill the trenches 245-S. A chemicalmechanical planarization (CMP) process is carried out to remove the topportion of the metal layer 251 and Ti/TiN layer 260 to form a planar topsurface. In FIG. 3K, a drain trench mask is applied to etch a draintrench 245-D in the semiconductor material, e.g. by oxide etch followedby silicon etch. The drain trench 245-D reaches down through the superjunction structure and the P-epi 215 to reach the N+ substrate 245. N+regions 255-1 are formed along the sidewalls of the drain trench 245-D.Another Ti/TiN layer 260-1 is formed on the sidewalls of the trench anda metal 251-1 if formed on the Ti/TiN layer 260-1. The metal 251-1 canbe CMP back on top, as shown in FIG. 2L. A second metal layer isdeposited on the top surface covering over the Ti/TiN layer 260. AnotherTi/TiN layer 261 may optionally be deposited before the metal layer 270is deposited. A metal mask is applied to pattern the metal layer into asource metal 270-S, gate metal (not shown) and optionally a top drainmetal (not shown), as shown in FIG. 3M. The gate 230 may be connected toa gate metal (not shown) in the third dimension. In FIG. 3N, apassivation layer 280 formed e.g., with a layer comprising a standardSiO2/nitride/polyimide stack followed by applying a passivation mask topattern the passivation layer 280 to expose the source metal 270-S.Then, a back grinding operation is performed followed by a back metalprocess to form a bottom drain metal layer 290 on the bottom surface ofthe P+ substrate 205 to complete the manufacturing processes.

There can be many alternative methods for forming the lateralsuperjunction JFET of this invention which should be obvious to oneproficient in the art. For example, there may be many ways to form gate,drain and source columns.

In an alternative method starting in FIG. 3A-1 , the super junctionstructure may be initially formed on a single P-type substrate 215′—theP type substrate 215′ would be similar to the P-epi layer 215 of FIG.3A, but does not include N+ substrate 205. The other processing stepsare the same up to the back-grinding step of FIG. 3O. At this point, theback side of P-type substrate 215′ is ground back to, or close to thebottom of drain trench 245-D, as shown in FIG. 3B-1 . The N+ dopant ionscan be implanted into the back side of the device to form N+ drain 205′,which connects to the drain trench 245-D, and a back drain metal 290 maybe formed. Some other alternative techniques are shown later in thespecification.

A JFET is a normally on device. In many applications a normally offpower switch is preferred over a normally on power switch for variousreasons including the device being off at circuit start-up,compatibility with existing designs, and familiarity. FIGS. 4A-4C showthree possible layouts of integrating a low voltage MOSFET with a highvoltage lateral super junction JFET of this invention together on asingle semiconductor die. The MOSFET can be arranged with the JFET in acascode configuration like the circuit shown in FIG. 4A in order to makethe device a normally off power switch device. In FIG. 4A, thesemiconductor die 390A contains a high voltage lateral super junctionJFET 391A of this invention and a low voltage MOSFET region 392A. TheJFET 391A and the MOSFET 392A each occupy a separate portion in the die390A. In FIG. 4B, three MOSFET regions 392B are distributed throughout asingle large JFET region 391A to reduce package resistance andinductance in a die 390B. In FIG. 4C, a MOSFET 392C is integrated at thedevice cell level of JFET 391C in a die 390C. As shown in FIG. 4C, aMOSFET 392C is integrated with each cell of JFET 391C. The MOSFETs andJFETs may be connected internally and/or externally in the cascodeconfiguration. The low voltage MOSFET is very well known in the art andcan come in any number of forms including lateral MOSFET, verticalMOSFET, trench gate, planar gate, etc., and there can be a number ofdifferent packaging schemes for connecting the MOSFET to the JFET in acascode circuit configuration.

Of course, a MOSFET can also be co-packaged with a lateral superjunctionJFET of this invention in a single package having separate MOSFET andsuperjunction JFET semiconductor dies. The semiconductor dies can beco-packaged side-by-side or in a stacked configuration.

FIG. 5A shows a cross section of a perspective view of a MOSFET 470integrated with a high voltage lateral super junction JFET 400 of thisinvention. In this embodiment, a MOSFET 470 is integrated into each JFET400 cell, like in the layout of FIG. 4C. In this layout, the lateral LowVoltage MOSFET 470 is placed in the 3rd dimension, perpendicular to theplane channel direction of the lateral Super-Junction JFET 400. The JFET400 is similar in structure to the lateral superjunction JFET 100 ofFIG. 2 . In these figures, the JFET N+ drain and source columns and JFETP+ gate columns are represented as simple N and P doped columns (420-D,420-S, 430). The doped columns 420-D, 420-S, and 430 operate in the sameway as the N and P columns of JFET 100. In this embodiment, the MOSFET470 comprises an N+ source region 471 in a P body region 472. The MOSFETN+ source 471 also acts as the overall source of the cascode circuit.The MOSFET 470 further comprises a planar gate 473 in oxide 475 whichalso acts as the overall gate of the cascode circuit. The N+ drain ofthe MOSFET 470 doubles as the N+ source column 420-S of the JFET 400.The lateral super junction structure of the JFET 400 comprises chargebalanced, alternating stacked P layers 440 and N layers 450, to form alateral super junction drift region running from the JFET N+ sourcecolumn 420-S to the JFET N+ drain column 420-D. The super junctionstructure is supported on a P epitaxial layer 415 over an N+ drainsubstrate 405. The drain column 420-D extends down to contact the N+drain substrate 405. A drain metal 410 may be located beneath the N+drain substrate 405. The JFET N+ drain column 420-D (and N+ drainsubstrate 405 and drain metal 410) act as the overall drain of thecascode circuit. The JFET 400 is controlled by the JFET P+ gate column430, which extends down through the super junction P and N layers 440and 450 to contact the P-Epitaxial layer 415. The JFET P+ gate column430 is shorted to the MOSFET N+ source region 471 by a source metal 488,in accordance to the cascode circuit. In accordance with this invention,an avalanche diode 421 is formed from the bottom of the JFET P+ gatecolumn 430 through the P-epitaxial layer 415 to the N+ drain substrate405. The avalanche diode 421 diverts avalanche current away from thesuper junction structure formed from charge balanced alternating stackedN and P layers 450 and 440.

As shown in the schematic of the circuit, the MOSFET (MOS) 470 isconnected to the superjunction JFET 400 in a cascode configuration. Thedrain of the MOS 470 is connected the JFET source 420-S. The JFET gate430 is connected to the MOS source 471. The lateral superjunction JFET400 further comprises an avalanche diode 421 between its gate 430 anddrain 420-D in accordance with this invention.

FIG. 5B is the same as FIG. 5A, but shows the location of vertical crosssections displayed in FIGS. 5D-F. FIG. 5D shows the cross section A-A,with the source metal 488 connected to both the MOS N+ source 471 andthe JFET P+ gate column 430. The source metal 488 also contacts the MOSP body 472 in a third dimension. FIG. 5E shows the cross section B-Bwith the N+ JFET source column 420-S and the N+ JFET drain column 420-D.The cross section B-B illustrates the lateral superjunction JFET 400,although the P+ JFET gate column 430 is in a third dimension. A P+implant is formed under the N+ JFET source column 420-S to suppress aparasitic NPN transistor formed from the N+ source column 420-S to theP-epitaxial 415 to the N+ drain substrate 405. FIG. 5F shows the crosssection C-C, which illustrates the low voltage MOSFET 470. MOSFETs arevery well known devices, and it should be apparent that other equivalentstructures may be formed to integrate a MOSFET with the lateral superjunction JFET of this invention.

A top view of the lateral super junction JFET 400 integrated with MOSFET470 of FIGS. 5A and 5B may be seen in FIG. 5C. For clarity, the topinsulating layers are not shown. An outline of the source metal 488 isindicated by the dashed-dot line. Dashed lines show the outlines ofcontact openings 489 for the source metal to make contact to the N+ MOSsource region 471 and the P+ JFET gate column 430 through the oxide 475(not shown). MOS gates 473 allow channels to be formed in the underlyingbody regions 472 (not shown) from the N+ MOS source regions 471 to theN+ MOS drain region/JFET source column 420-S. A polysilicon gate runner477 connects the gates 473 together. From the N+ JFET source column420-S, the current can flow through the super junction structure (the Nlayers 450 under top P layer 440) to the N+ JFET drain column 420-D. Inaccordance with the cascode configuration, the JFET P+ gate column 430is shorted to the MOS N+ source region 471 at the contact openings 489and provides high voltage blocking for the device when the MOS gate 473is turned off.

FIG. 6A shows a top view of a bottom drain lateral superjunction JFET500 of this invention. FIG. 6A shows the JFET 500 in a stripeconfiguration. It has an N+ JFET source column 520-S which is formed asa stripe. Running alongside the N+ JFET source column 520-S stripe areP+ JFET gate columns 530 formed intermittently like a dashed stripe. TheP+ JFET gate columns 530 are staggered so that a current path isavailable from the N+ JFET source column 520-S to the N+ JFET draincolumn 520-D. The cross section D-D of FIG. 6A is shown in FIG. 6A-1 .An N+ substrate 505 supports a P-epi layer 515. Over the P-Epi layer,the lateral superjunction structure is formed from alternating stacked Players 540 and N layers 550. The P layers 540 and N layers 550 areformed with doping concentrations and thicknesses selected such thatthey are charge balanced. The N layers 550 form lateral paths from theN+ JFET source column 520-S to the N+ JFET drain column 520-D. The P+JFET gate columns 530 pinch off the current. The N+ JFET drain column520-D extends down to the N+ drain substrate 505, to form a bottom draindevice. A P+ region 565 may be formed under the N+ JFET source column520-S to suppress a parasitic NPN bipolar transistor action from the N+source column 520-S to the P-epi 515 to the N+ substrate 505. Anavalanche diode 521 may also be formed from P+ gate column 530 to the N+substrate (by way of P-epi 515) in accordance with this invention.

The individual gate columns 530 may be connected together with a shallowP+ surface implant 541, as shown in cross section E-E in FIG. 6A-2 .Alternatively, a metal strip could connect the P+ gates 530 on the top.FIG. 6A-3 shows a completed JFET 500-3 with source metal and gate metalon the top, and drain metal on the bottom taken along cross section lineD-D of FIG. 6A. A source metal 521 contacts the N+ JFET source column520-S, and a gate metal 531 contacts the P+ JFET gate column 530 throughopenings in a thick dielectric layer 544, e.g. oxide or BPSG(borophosphosilicate glass). A drain metal 510 is formed on the bottomof the N+ substrate 505.

A low voltage MOSFET can also be integrated with the lateralsuperjunction JFET 500 such that it is in the same plane as thesuper-junction JFET. The MOSFET may be connected to the JFET 500 in acascode circuit configuration to make the overall device a normally offdevice. FIG. 6B is a top view showing the next step after FIG. 6A offorming the MOSFET—forming a MOS gate electrode 573, e.g. polysilicon(poly), on top of the device. A thin gate oxide (not shown in FIG. 6B)insulates the MOS gate 573 from the semiconductor surface.

Next, N+ MOS source 571, and N+ MOS drain 575 regions are implanted intothe top of the device, self-aligned to the MOS gate 573. A P MOS bodyregion 572 is also formed self-aligned to the MOS gate 573, as shown inthe top view of FIG. 6C. The outlines of the P+ JFET gate columns 530and N+ JFET source column 520-S are indicated by dashed lines.

The cross section F-F of FIG. 6C is shown in FIG. 6C-1 . The N+ MOSsource 571 is located within P MOS body region 572, with both beingself-aligned to the MOS gate 573. The MOS gate 573 is separated from thesemiconductor surface by a thin gate dielectric 574, e.g. gate oxide. Onthe other side of the MOS gate 573, the N+ MOS drain 575 is also formedself-aligned to the MOS gate 573. The N+ MOS drain 575 is connected tothe N+ JFET source column 520-S in accordance to the cascode circuitconfiguration. A source metal 580 contacts the N+ MOS source 571 and PMOS body 572 through openings in a thick dielectric layer 576. The P MOSbody 572 connects to the P+ JFET gate column 530, so the source of theMOS and the gate of the JFET are connected, in accordance with thecascode circuit configuration. An electric current can thus start at theN+ MOS source 571, and flow to the N+ MOS drain 575, under the controlof the MOS gate 573. From the N+ MOS drain 575, the current can continuethrough the N+ JFET source column 520-S to the superjunction N layers550 and flow laterally to the N+ JFET drain 520-D. The P+ JFET gate 530is shorted to the N+ source 571 in the cascode configuration, so whenthe MOSFET is on, the JFET gate 530 is approximately at the samepotential as JFET source 520-S (and MOS drain 575), thereby allowingcurrent flow in the N layers 550. The current can flow down the N+ JFETdrain column 520-D to the N+ drain substrate 505, and to the drain metal510 below. When the MOSFET is off, the JFET gate 530 is at a lowerpotential than the JFET source 520-S/MOS drain 575 thus shutting offcurrent flow as will be explained. Removal of the MOS gate 573 biasturns off the low voltage MOSFET. In this mode, application of apositive bias to the drain terminal 510 will result in a positive biason the N layers 550 and a corresponding negative bias on the JFET P gate530 causing the superjunction N layers 550 and P layers 540 and theP-epitaxial 515 to deplete. At a certain drain bias, the depletionregions from adjacent P layers 540 merge, and pinch-off the JFET sourcecolumn 520-S from additional drain voltage. This allows the device toblock high voltages with low leakage. It also allows a low voltageMOSFET to be used in the cascode circuit, because the JFET supports theadditional voltage after pinch off. The charge in the P epi region 515created when it depletes at high drain biases reduces the peak electricfield under the P+ gate 530 and allows this device structure to have ahigh substrate breakdown voltage. Though not shown here, the MOS gate573 may be connected to a gate metal terminal located on another part ofthe semiconductor die. The source and drain stripe structures and layoutmay be repeated throughout a semiconductor die, as shown in FIG. 6D.

FIG. 7A shows a top view of an alternative layout for the device aclosed cell layout. More specifically, it shows a hexagonal closed celllayout. The basic lateral superjunction JFET 500′ structure is shown inthe top view of FIG. 7A and the cross section view of FIG. 7A-1 . The N+JFET source columns 520′-S are arranged in an interconnected hexagonalweb. Alongside the N+ JFET source column 520′-S are the P+ JFET gatecolumns 530′. The P+ JFET gate columns 530′ are staggered to allowcurrent flow between them. In the center of each hexagon is the N+ JFETgate column 520′-D. As shown in the cross section view G-G of FIG. 7A-1, the JFET elements are located in the superjunction structure of chargebalanced, stacked P layers 540 and N layers 550 over a P-Epi 515 over anN+ substrate 505. In each hexagonal closed cell, the current flowslaterally from the N+ JFET source column 520′-S at the sides of thehexagon to the N+ JFET drain column 520′-D at the center of the hexagonthrough the super junction N layers 550. The current can then flow downthe N+ drain column 520′-D to the N+ substrate 505. An NPN suppressingP+ region is formed under the N+ JFET source column 520′-S and anavalanche diode 521′ is formed from the P+ JFET gate column 530′ to theN+ substrate 505.

As with the stripe configuration of FIGS. 6A-6D, a MOSFET may be easilyintegrated with the JFET 500′ of FIG. 7A in a cascode circuitconfiguration, as shown in FIGS. 7B-7C. In the top view of FIG. 7B,which is a close-up view of a single hexagonal closed cell, a network ofMOS gate electrodes 573′ are formed over the top of the die. The MOSgates 573′ run alongside the N+ JFET source columns 520′-S.

In the top view of FIG. 7C, N+ MOS source 571′ and N+ MOS drain 575′regions are formed self-aligned to the MOS gate 573′. P MOS body 572′regions are also formed self-aligned to the MOS gate 573′, surroundingthe N+ MOS source regions 571′. The outlines of the N+ JFET source520′-S and P+ JFET gate 530′ are shown as dashed lines. As shown in thecross section H-H of FIG. 7C-1 , the source metal 580′ contacts the N+MOS source 571′ and the P MOS body 572′ through openings in the thickdielectric 576′. The source metal 580′ is also connected to the P+ JFETgate column 530′ through P MOS body 572′, in accordance with the cascodecircuit. The N+ MOS drain 575′ is formed on the other side of the MOSgate 573′. A thin gate dielectric 574′ insulates the MOS gate 573′ fromthe semiconductor surface. The N+ MOS drain 575′ is connected to the N+JFET column 520′-S in accordance with the cascode circuit configuration.

FIGS. 8A-7Q are a series of cross sectional views showing a method ofmanufacturing a MOSFET integrated with a bottom drain lateralsuperjunction JFET of this invention. The process begins in FIG. 8A withforming a lateral super junction structure made of alternating stackedcharge balanced P type layers 740 and N type layers 750. The lateralsuper junction structure is formed over the P-epi layer 715 which isformed over the N+ substrate 705.

Next a hard mask 722, e.g. oxide, is formed and patterned on top. A gatetrench 725 is etched into the semiconductor material, as shown in FIG.8B. The gate trench 725 extends down through the lateral super junctionstructure to reach the P-epi 715. In FIG. 8C, the gate trench 725 islined with P+ implant, e.g. by angled implant, to from the P+ gatecolumn 730. In FIG. 8D, a second hard mask 726 is formed and patterned.The second hard mask material may also fill in the gate trench 725,e.g., with an oxide fill 727. Source trench 745-S and interim draintrenches 745-D are etched into the semiconductor material through thelateral super junction structure of P layers 740 and N layers 750 to theP-epi 715. The interim drain trenches 745-D are much narrower than thesource trench 745-S. The sidewalls of the source trench 745-S andinterim drain trenches 745-D are implanted N type to form N type sourcecolumn 720-S and interim N type drain columns 720-D. In FIG. 8E, N+polysilicon spacers 752 are formed along the sidewalls of the sourcetrench 745-S. By way of example, the polysilicon spacers 752 may beformed by depositing a layer of polysilicon, and then isotropicallyetching the polysilicon layer to leave the polysilicon spacers 752 insource trench 745-S. The process of forming polysilicon spacers 752additionally fills in the narrow interim drain trenches 745-D with N+polysilicon fill 753. In FIG. 8F, a vertical P+ implant is performed toform a P+ region 765 in the P-epi layer 715 at the bottom of the sourcetrench 745-S. The P+ region 765 suppresses the parasitic NPN transistorformed from the N source column 720-S to the P-epi 715 to the Nsubstrate 705.

In FIG. 8G, oxide 723 is deposited over the device, and then planarized.The oxide also fills in the remaining space in the source trench 745-S.The oxide 723 is patterned with a third mask and etched down to thesurface of the semiconductor material in the regions where the MOSFETwill be formed. A thin gate oxide 774 is formed on the exposedsemiconductor material, and then a gate poly 773 is formed and patternedwith a fourth mask on top of the gate oxide 774, as shown in FIG. 8H. InFIG. 8I, a fifth mask is used as a body block mask. A P body implant isperformed to form P-body region 772, using the body block mask 719, gatepoly 773, and oxide 723 as a mask. The body region 772 is self-alignedto the gate poly 773. If the gate poly 773 is long enough, e.g. if thegate poly 773 extends to the N source column 720-S—the body block mask719 may not even be necessary.

Next, in FIG. 8J, a blanket N+ implant is performed to form N+ MOSsource region 771 and N+ MOS drain region 775 on opposite sides of thegate poly 773. The gate poly 773 forms a planar gate between the MOSsource 771 and the MOS drain 775. The MOS source 771 and MOS drain areself-aligned to the gate poly 773. The N+ MOS drain 775 connects to theN JFET source column 720-S and the P-body region 772 connects to the P+JFET gate column 730.

An insulating material such as BPSG (borophosphosilicate glass) 776 ifformed on top, insulating the top surface of the device, as shown inFIG. 8K. In FIG. 8L, a sixth mask is used to form a contact hole 777 byetching through the BPSG 776 and into the semiconductor material. Thecontact hole 777 should etch through the N+ MOS source 771 to reach theP-body 772. A P+ body contact region 778 can be formed at the bottom ofcontact hole 777, e.g. by vertical implantation. As shown in FIG. 8M, asource metal 780 is deposited and patterned such that the source metal780 fills the contact hole 777 and makes contact to the N+ MOS source771 and P-body 772 (via P+ body contact region 778. The source metal 780is also connected to the P JFET gate column 730 through the P body 772,in accordance with the cascode circuit configuration. In the thirddimension, a gate pad (not shown) may also be formed for externalconnection to the MOS gate poly 773. By way of example, the source metal780 may be aluminum.

In FIG. 8N, an eighth mask is used to form the drain trench 781 at theinterim JFET drain columns 720-D. The drain trench 781 is etched throughthe interim JFET drain columns 720-D, down through the P-epi 715 andinto the N+ substrate 705. In FIG. 8O, the drain trench 781 is linedwith N type regions 779, e.g. by forming N+ polysilicon spacers, N+epitaxial growth, or by the exposed sidewalls of the trench. Metal 783is formed in the drain trench 781, as shown in FIG. 7P. By way ofexample metal 783 may be copper plating that forms in the drain trenchas metal 783 and on top of the source metal 780 as metal 782. Finally inFIG. 7Q, the back side of the wafer/N+ substrate 705 is thinned and abottom drain metal 790 is formed on the bottom.

As can be seen, there are many ways of forming the JFET source, gate anddrain columns including doping the semiconductor, etching a trench thenfilling with doped polysilicon, etching a trench and lining the trenchwith implanted dopants or doped polysilicon and filling the rest of thetrench with conductive material or oxide, and so on. More alternativemethods may include etching a trench a forming doped side walls byepitaxial growth, from the top surface implantation, implantations whileforming the epitaxial layer and super junction layers, etc.

While the above embodiments may have been described for silicon, itshould be clear to one of ordinary skill in the art that they can beapplied to any semiconductor material such as silicon carbide (SiC),germanium (Ge), diamond, or gallium arsenic (GaAs) or gallium nitride(GaN). Also, it should be clear that the N+ and P+ columns comprisingthe source, drain and gate columns may be formed in a number ofdifferent ways. Although the present invention shows an N-channel JFET,it may also be applied to a P-channel JFET, by switching theconductivity types of the P type and N type semiconductor regions.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter reading the above disclosure. Accordingly, it is intended that theappended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

We claim:
 1. A semiconductor power device comprising: a plurality oftransistor close cells disposed on a semiconductor substrate including asuper junction structure disposed near a top surface of thesemiconductor substrate wherein the super junction structure comprisinga plurality of laterally stacked layers of alternating conductivitytypes of a first and second conductivity types extending laterallythrough each close cell; a central vertical column of a firstconductivity type disposed at a center of each close cell and extenddownward through the super-junction structure; a plurality of gatecolumns of a second conductivity type separated from each othersurrounding the first column extending downward through the superjunction structure; and a peripheral vertical column of the firstconductivity type surrounding the plurality of the gate columnsconstitutes a ring enclosing each of the transistor close cells.
 2. Thesemiconductor power device of claim 1, wherein: the semiconductorsubstrate further comprises a bottom semiconductor layer of a firstconductivity type, wherein the first column extends downwardly toconnect to the bottom semiconductor layer.
 3. The semiconductor powerdevice of claim 2, wherein: the central vertical column functioning as adrain and the peripheral vertical column functioning as a source for thetransistor closed cells.
 4. The semiconductor power device of claim 2wherein: the semiconductor substrate further comprises an intermediatesemiconductor layer of a second conductivity type disposed under thesuper junction structure and on top of the bottom semiconductor layer.5. The semiconductor power device of claim 4 wherein: the gate columnextends downwardly into the intermediate semiconductor layer toconstitute a built-in gate-drain avalanche clamp diode from acombination of the bottom semiconductor layer through the intermediatesemiconductor layer to the gate column.
 6. The semiconductor powerdevice of claim 4 wherein: the source column extends into theintermediate semiconductor layer and further comprises a bipolarsuppressing region in the intermediate semiconductor layer at the bottomof the source column; the bipolar suppressing region is doped with thesecond conductivity type.
 7. The semiconductor power device of claim 3wherein: the gate column extends deeper than the source column.
 8. Thesemiconductor power device of claim 5 wherein: the source, drain andgate columns form a JFET, and wherein the source columns, thesemiconductor power device further comprises a MOSFET connected in acascode circuit configuration with the JFET.
 9. The device of claim 8,wherein: the MOSFET further comprises a source region, a body region, agate, and a drain region wherein the source region extends along adirection parallel to the source column and is separated therefrom withthe body region disposed between the source region and the sourcecolumn.
 10. The semiconductor power device of claim 8 wherein: theMOSFET is integrated with the JFET at the device cell level.
 11. Thedevice of claim 1, wherein: the peripheral vertical column enclosing thetransistor closed cell is configured substantially as a hexagonal-shapedring.
 12. The device of claim 8 wherein: the gate of the MOSFET isconfigured to form an inversion channel between the source region of theMOSFET and the source column.
 13. A semiconductor power devicecomprising: a plurality of transistor close cells disposed on asemiconductor substrate including a super junction structure disposednear a top surface of the semiconductor substrate wherein the superjunction structure comprising a plurality of laterally stacked layers ofalternating conductivity types of a first and second conductivity typesextending laterally through each close cell; a central vertical columnof a first conductivity type disposed at a center of each close cell andextend downward through the super-junction structure; a plurality ofgate columns of a second conductivity type separated from each othersurrounding the first column extending downward through the superjunction structure; a peripheral vertical column of the firstconductivity type surrounding the plurality of the gate columnsconstitutes a ring enclosing each of the transistor close cells; and anetwork MOS gate electrodes disposed on top of and configured to have asubstantially same shape as the ring as the peripheral vertical columnsand further having interconnecting electrodes connected between the MOSgate electrodes of adjacent transistor closed cells.
 14. Thesemiconductor power device of claim 13, wherein: the central verticalcolumn functioning as a drain and the peripheral vertical columnfunctioning as a source for the transistor closed cells.
 15. Thesemiconductor power device of claim 14 wherein: the source, drain andgate columns form a JFET, and wherein the source columns, thesemiconductor power device further comprises a MOSFET connected in acascode circuit configuration with the JFET.
 16. The device of claim 15,wherein: the MOSFET further comprises a source region, a body region, agate, and a drain region wherein the source region extends along adirection parallel to the source column and is separated therefrom withthe body region disposed between the source region and the sourcecolumn.
 17. The semiconductor power device of claim 15 wherein: theMOSFET is integrated with the JFET at the device cell level.
 18. Thedevice of claim 13, wherein: the peripheral vertical column togetherwith the networked gate electrodes enclosing the transistor closed cellis configured substantially as a hexagonal-shaped ring.
 19. The deviceof claim 15 wherein: the gate of the MOSFET is configured to form aninversion channel between the source region of the MOSFET and the sourcecolumn.